Serpin family protein protease inhibitors function as key regulators of blood coagulation proteases. Two serpins, anti-thrombin and protein Z-dependent protease inhibitor (ZPI), are known to inhibit procoagulant proteases in a manner that is regulated by cofactors and dependent on the functional state of the proteases, but the molecular details of this complex regulatory mechanism are poorly understood. The physiologic importance of these serpins in regulating coagulation proteases is borne out by the observations that knocking out the mouse genes results in embryonic lethality due to a consumptive coagulopathy in the case of anti-thrombin and increased thrombosis when mice are given a thrombotic challenge or are bred on a factor V Leiden background in the case of ZPI. The long-term goal of our studies is to understand how coagulation proteases are regulated by these two critical serpin inhibitors. Our proposed studies seek to build on our prior studies to advance the detailed molecular understanding of this cofactor-dependent regulation. With respect to anti-thrombin, our recent studies have suggested an important revision of the allosteric mechanism of activation of this serpin by heparin in showing that activation is mediated principally by the relief of repulsive interactions with factor Xa and factor IXa and secondarily by the relief of structural constraints of the reactive site that promote reactive site and exosite interactions with the protease. Our proposed studies seek to characterize the molecular determinants of the repulsive interactions, establish how certain mutations can decouple the structural changes accompanying activation to produce an intermediate activated state and to show how an induced-fit mechanism of heparin binding is coupled to allosteric activation. With respect to ZPI, our recent X- ray structure of a complex of ZPI with its cofactor protein, protein Z, has provided new insights into the mechanism by which the cofactor promotes the specific inhibition of membrane-associated factor Xa, despite the unfavorable P1 Tyr of ZPI. Our studies seek to map putative exosites on ZPI for factor Xa and factor XIa, validate the ZPI sites for protein Z binding that have been revealed by the X-ray structure, and assess the role of the unique N-terminal tail of ZPI in the inhibition of membrane-associated factor Xa. For both serpins, gain of function studies in which the serpin scaffold of 11-proteinase inhibitor is used to graft the molecular determinants of exosite interactions in anti-thrombin and of both protein Z and exosite interactions in ZPI will provide a stringent test of the minimal determinants of protease and cofactor recognition by these serpins.)